Method of erasing a flash memory cell

ABSTRACT

Methods are disclosed for erasing a flash memory cell including: (a) a semiconductor substrate, (b) a gate, (c) a source, (d) a drain, (e) a well, the gate including: (1) a tunnel oxide film, (2) a floating gate, (3) a dielectric film and (4) a control gate stacked on the semiconductor substrate. In one of the disclosed methods, a negative bias voltage is applied to the control gate, the source and drain are floated, a positive bias voltage is applied to the well to thereby create a positive bias voltage in the source and the drain, a ground voltage is applied to the well at a first time while maintaining the negative bias voltage a the control gate; and subsequently a ground voltage is applied to the control gate.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a Divisional of U.S. patent application Ser. No.10/364,137 filed Feb. 11, 2003 now U.S. Pat. No. 6,934,193, which is aDivisional of U.S. patent application Ser. No. 09/896,663 filed Jun. 29,2001 now U.S. Pat. No. 6,545,914, which claims priority from KoreanPatent Application Serial No. 2000-36519 filed Jun. 29, 2000 which areall incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates generally to a method of erasing a flashmemory cell, and more particularly, to a method of erasing a flashmemory cell having a stacked gate structure at an increased rate whileminimizing the loss of stored data.

BACKGROUND OF THE INVENTION

Referring to FIG. 1, a flash memory cell generally includes a gate inwhich a tunnel oxide film 2, a floating gate 3, a dielectric film 4 anda control gate 5 are stacked on a channel of a semiconductor substrate1. A source 6 and a drain 7 are formed in the semiconductor substrate 1on either side of the gate.

The flash memory cell is typically programmed or erased by applying abias voltage condition to the semiconductor substrate 1, the controlgate 5, the source 6 and the drain 7. To program the flash memory cell,hot electrons are injected into the floating gate 3. On the other hand,to erase the flash memory cell, electrons injected into the floatinggate 3 are discharged via the semiconductor substrate 1.

Erasing a flash EEPROM device typically involves changing all of theflash memory cells constituting a chip to the same state. In otherwords, the threshold voltages of all of the flash memory cells arechanged to the same state.

The threshold voltage of a flash memory cell generally depends on theamount of charges injected into the floating gate 3. Therefore, in orderto change the threshold voltages of all of the flash memory cells intothe same state, electrons injected into the floating gate 3 aresimultaneously discharged, a process which is usually performed usingthe prior art Fowler-Nordheim (F-N) tunneling method. As explainedbelow, the use of the prior art F-N tunneling method overcomes a numberof problems.

First, as electrons injected into a floating gate 3 typically have verylow kinetic energy, they are unable to jump over a potential barrier ofabout 3.1 eV in the tunnel oxide film 2.

Second, although the electrons injected into the floating gate 3 of theflash memory cell are simultaneously discharged during the erasureoperation, power consumption is minimized as the current flow is limitedto the discharge of the electrons from the floating gate 3.

In the case of a flash EEPROM device having a sub-quarter design rule,that is, 0.25 μm, an erasure operation is performed using the prior artchannel F-N tunneling method, as shown in FIG. 2.

FIG. 2 shows a flash memory cell having a triple well structure. Using aprior art method to erase the flash memory cell, a negative bias voltage(−V) is applied to a control gate 5, a positive bias voltage (+V) isapplied to a P-well 1 a and to a N-well 1 b of a semiconductor substrate1, while the source 6 and the drain 7 are floated. This results in theelectrons (σ), previously injected into the floating gate 3, beingdischarged via the semiconductor substrate 1. Using the prior artchannel F-N tunneling effect causes the tunneling region to become widerthan when using a prior art junction erasure effect. As the dopantconcentration of the channel becomes uniform, the number of flash memorycells that are erased early as a result of the induced electromagneticfield is reduced, so that the distribution of the threshold voltagesafter the implementation of the prior art erasure operation becomesconstant. More specifically, as the dopant concentration of the channeland the well is increased, the number of holes that accumulate on thesurface of the semiconductor substrate and the distribution of theelectromagnetic field become uniform. As a result, the distributiondegree becomes more constant. Also, as the speed is increased, thenumber of flash memory cells that are slowly erased is reduced.Therefore, the source 6 and the drain 7, that is, the junction region,is floated.

Referring to FIG. 3, using the prior art F-N tunneling effect, typicallyrequires that a high electromagnetic field of more than 10 MV/cm beformed at both sides of the tunnel oxide film 2. Therefore, a highnegative potential must be applied to the floating gate 3. In addition,the thickness of the dielectric film 4 must be reduced in order toincrease the coupling capacitance (Cfg) between the control gate 5 andthe floating gate 3, to which a negative bias voltage (−V) is applied.

The electromagnetic field (E) may be determined using Equation 1, whereV is the applied voltage and t is the thickness of a dielectric film.Therefore, the thickness of the tunneling oxide film 2, acting as adielectric film between the floating gate 3 and the channel, must bereduced.E=V/t  Equation 1

However, if the thickness of the dielectric film existing on and belowthe floating gate 3 is reduced, a number of problems may occur.

The flash EEPROM device is a nonvolatile memory device which stores databy injecting hot electrons into the floating gate 3. The stored data isoften required to be stored for a period of ten or more years. Anelectromagnetic field is typically formed toward the direction of thefloating gate 3 if hot electrons are over-charged into the floating gate3. If the thickness of the dielectric film is too thin, electrons flowout of the floating gate as shown in FIG. 4. As a result, stored datamay be transformed or lost.

Typically, if the thickness of the dielectric film is reduced, the speedof the erasure operation is increased but the conservationcharacteristic of the stored data is degraded. Therefore, there is aneed for a new method of increasing the speed of the erasure operationwhile maintaining the thickness of the dielectric film.

SUMMARY OF THE INVENTION

In accordance with an aspect of the invention, a method is provided forerasing a flash memory cell including: (a) a semiconductor substrate,(b) a gate, (c) a source, (d) a drain and (e) a well, where the gateincludes: (1) a tunnel oxide film, (2) a floating gate, (3) a dielectricfilm and (4) a control gate stacked on the semiconductor substrate. Themethod includes the steps of floating the source and the drain, applyinga negative bias voltage to the control gate, applying a positive biasvoltage to the well to thereby create a positive bias voltage in thesource and the drain, applying a ground voltage to the well at a firsttime while maintaining the negative bias voltage a the control gate andsubsequently applying a ground voltage to the control gate.

In accordance with an alternative aspect of the invention, a method isprovided for erasing a flash memory cell including: (a) a semiconductorsubstrate, (b) a gate, (c) a source, (d) a drain and (e) a well, wherethe gate includes: (1) a tunnel oxide film, (2) a floating gate, (3) adielectric film and (4) a control gate stacked on the semiconductorsubstrate. The method includes the steps of applying a negative biasvoltage to the control gate, applying a positive bias voltage to thewell, creating a positive bias voltage in the source and the drain,applying a ground voltage to the control gate and simultaneouslyfloating the well, the source and the drain.

In accordance with another aspect of the invention, a method is providedfor erasing a flash memory cell including: (a) a semiconductorsubstrate, (b) a gate, (c) a source, (d) a drain, (e) a P-well formed inthe semiconductor substrate and (f) a N-well formed in the semiconductorsubstrate, where the gate includes: (1) a tunnel oxide film, (2) afloating gate, (3) a dielectric film and (4) a control gate stacked onthe semiconductor substrate. The method includes the steps of floatingthe N-well, the source and the drain, applying a negative bias voltageto the control gate, applying a positive bias voltage to the P-well,applying a ground voltage to the P-well, applying a ground voltage tothe control gate, creating a ground voltage at the source and the drainand applying a ground voltage to the N-well.

In accordance with yet another aspect of the invention, a method isprovided for erasing a flash memory cell including: (a) a semiconductorsubstrate, (b) a gate, (c) a source, (d) a drain, (e) a well, where thegate includes: (1) a tunnel oxide film, (2) a floating gate, (3) adielectric film and (4) a control gate stacked on the semiconductorsubstrate. The method includes the steps of floating the source and thedrain, applying a positive bias voltage to the control gate, applying anegative bias voltage to the well to thereby create a negative biasvoltage in the source and the drain, applying a ground voltage to thewell at a first time while maintaining the positive bias voltage at thecontrol gate and subsequently applying a ground voltage to the controlgate.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of a prior art flash memory cell;

FIG. 2 is a cross-sectional view of a prior art flash memory cell havinga triple well structure;

FIG. 3 is a segmented cross-sectional view of a prior art flash memorycell illustrating the presence of coupling capacitance within the flashmemory cell;

FIG. 4 is a cross-section view of a prior art flash memory cellillustrating the discharge path of the previously injected electronsfrom the floating gate when the thickness of the dielectric film is toothin;

FIG. 5 is a graphical representation of the bias voltage applied to thecontrol gate, to the P-well, to the source and to the drain in anexemplary erasure method performed in accordance with the teachings ofthe invention;

FIG. 6 is a graphical representation of the change in the potential inthe junction region and in the P-well based on the application of thebias voltages illustrated in FIG. 5.

FIG. 7A and FIG. 7B are cross-sectional views of a flash memory cellillustrating a pathway of carriers in response to the application of thebias voltages illustrated in FIG. 5.

FIG. 8 is a graphical representation comparing the erasure time resultsfrom using a prior art method of erasing a flash memory cell and theerasure time results from using an exemplary erasure method performed inaccordance with the teachings of the invention.

FIG. 9 is a graphical representation of the substrate current and thecontrol gate current as a function of the junction bias voltage duringan exemplary erasure method performed in accordance with the teachingsof the invention;

FIG. 10 is a graphical representation of changes in threshold voltage asa function of erase time for variations in the the bias voltage appliedto the well;

FIG. 11 is a graphical representation of the leakage current as afunction of junction potential with a well of a triple well structureflash memory cell floated and with a junction region floated.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A preferred method of erasing a flash memory cell in accordance with theteachings of the invention will be described in detail with reference tothe accompanying drawings, in which like reference numerals are used toidentify same or similar parts.

Referring to FIGS. 7A and 7B, an NMOS flash memory cell generallyincludes a semiconductor substrate 1, a stacked gate, a source 6, adrain 7, a junction region that is formed between the source 6 and thedrain 7 and a P-well 1. The stacked gate includes a tunnel oxide film 2,a floating gate 3, a dielectric film 4 and a control gate 5 stacked onthe semiconductor substrate 1. The well region doped with P-type dopantsand the junction region doped with N-type dopants. The junction regiontypically has a dopant concentration that is greater than 1 E20/cm³ andthe well region typically has a dopant concentration falling within arange of approximately 1 E16/cm³ to approximately 1 E18/cm³. The tunneloxide film 2 typically has a thickness that falls within a range ofapproximately 60 Δ to approximately 150Δ. Referring to graph in FIG. 5,the flash memory cell erasure operation begins with floating the source6 and the drain 7 at 0 volts and applying a negative bias voltage (−V)to the control gate 5. Shortly thereafter, a positive bias voltage (+V)ranging from approximately five volts to approximately 12 volts isapplied to the P-well 1. P-N junctions are formed between the betweenthe P-well 1 and the source 6 and between the P-well 1 and the drain 7.The free electrons and the free holes are drawn to interfaces betweenthe source 6 and the P-well 1 and between the drain 7 and the P-well 1such that a depletion region is formed where only charges having nomovement remain. The width of the depletion region formed at each of theP-N junctions can be determined using Equations 2 and 3 provided below.Specifically, Equation 2 is used to determine X_(p), the width of thedepletion region at the P-well, and Equation 3 is used to determineX_(n), the width of the depletion region at the junction region, where εis the dielectric constant, q is the charge of a single electron, N_(d)is the donor concentration, N_(a) is the acceptor concentration andV_(i) is the built-in potential.X _(p)=+[(2ε/q)*(N _(d) /N _(a))*{1/(N _(a) +N _(d))}*V _(i)]  Equation2X _(n)=+[(2ε/q)*(N _(a) /N _(d))*{1/(N _(a) +N _(d))}*V _(i)]  Equation3

The width of the total depletion region (W) can be determined usingEquation 4.W=X _(p) +X _(n)=+[(2ε/q)*(1/N _(d)+1/N _(a))*V _(i)]®+[(2ε/q)*(1/N_(a))*Vi]  Equation 4

Since each side of the depletion layer is at an opposite charge withrespect to each other, each side can be viewed as the plate of acapacitor and the depletion region forms a junction capacitor acting asa dielectric in the form of a general parallel plate capacitor. Thejunction capacitance (C_(j)) can be determined using Equation 5.C _(j) =εA/W=(A/2)*+[(2qε*N _(a) *V _(i))  Equation 5

The implementation of the erasure operation employs the junctioncapacitance. As mentioned previously, prior to the application of thebias voltages to the control gate 5 and the P-well 1, a bias voltage of0 volts is applied to the source 6 and to the drain 7. The source 6 andthe drain 7 are then floated such that the potential of the source 6 andthe drain 7 are both maintained at a ground potential (0 volts). Whenthe positive voltage (+V) is then applied to the P-well 1, and forwardP-N junction diodes are formed between the source 6 and the P-well 1 andbetween the drain 7 and the P-well 1. The holes, which constitute themajority of the carriers in the P-well 1, are drawn towards the source 6and the drain 7 and the potential at the source 6 and at the drain 7increase in a positive direction.

The potential at the source 6 and at the drain 7 increase taking intoaccount the built-in potential difference of the P-N junction in anequilibrium state.

Since in a forward P-N junction diode, the built-in potential istypically about 0.7 volts, the source 6 and the drain 7 maintain a biasvoltage of about 0.7 volts less than the bias voltage that is applied tothe P-well 1. For 20 example, if the bias voltage applied to the P-well1 is approximately 9 volts, the source 6 and the drain 7, being afloating state, maintains a potential of about 8.3 volts. As result, themajority of the electrons previously injected into the floating gate 3are discharged.

Next, a voltage of 0 volts is applied to the P-well 1 such that thepotential at the P-well 1 is reduced to 0 volts within a very shortperiod of time. Unlike the P-well 1, the potential at the source 6 andthe drain 7 is not discharged instantly by the charge storage junctioncapacitor but is discharged gradually, as shown in FIG. 6. In FIG. 6,the lines W1 and W2 illustrate changes in potential in the P-well 1 andin the junction region of a flash memory cell having a well with atriple structure where the junction region is floated and a bias voltageof 0 volts is applied to the P-well 1. Specifically, line W1 indicatesthe potential change in the P-well 1 and the line W2 indicates thepotential change in the junction region.

Electromagnetic fields are created based on the potential differencesbetween the source 6 and the P-well 1 and between the drain 7 and theP-well 1 forcing the holes to leave the source 6 and the drain 7 untilthe potential at the source 6 and the drain 7 are pulled down to 0volts.

If a voltage of 0 volts is applied to the source 6 and the drain 7before the potential at the P-well 1 reaches 0 volts, a forward biascondition is created whereby, a large current flow is created instantly.Such a condition can critically damages the source 6 and the drain 7.Therefore, a potential of 0 volts is applied to the P-well 1, which inturn causes the source 6 and the drain 7 to be gradually pulled down to0 volts, so that a large electromagnetic field can be formed between thejunction region and the well thereby preventing the damagingly largecurrent flow.

As the size of the flash memory cell is reduced, a punch-throughphenomenon is generated. The dopant concentration of the well must beincreased in order to prevent the punch through phenomenon. In otherwords, as N_(a), the acceptor concentration, is increased, the width ofthe depletion region is reduced, which further increases the junctioncapacitance and thus increases the electromagnetic field.

The control gate 5 to which a negative bias (−V) is applied is removedsome time period after the P-well 1 has been discharged, as shown inFIG. 5, so that, prior to grounding the control gate 5, electromagneticfields are vertically formed between the source 6 and the control gate 5and between the drain 7, and the control gate 5. As the electromagneticfields, thus formed, form corner electromagnetic fields along with theside electromagnetic fields formed between the source 6 and the P-well 1a and between the drain 7 and the P-well 1, numerous hot holes and hotelectrons are generated as a result of a junction avalanche breakdown orband-to-band tunneling. As the hot holes thus generated flow over thebarrier potential (4.3 eV) of the tunnel oxide film, the direction ofthe hot holes are changed by the vertically formed electromagnetic fieldand are, thus, injected into the floating gate 3, which removeselectrons by offset of the potential due to the recombination of the hotholes with the injected electrons. The recombination process facilitatesthe performance of the erasure operation. That is, as shown in FIG. 7Aand FIG. 7B, the recombination of the holes and the electrons, which areinjected in a state in which the electrons are discharged from thefloating gate by the F-N tunneling effect promotes the erasure operationto thereby reduce erasure time.

Referring to FIG. 8, a line A1 indicates a change in the thresholdvoltage of a flash memory cell as a function of the erasure time when aprior art erasure method is used, and a line A2 indicates a change inthe threshold voltage of a flash memory cell as a function of theerasure time when an exemplary erasure method in accordance with theteachings of the invention is used.

FIG. 9 shows that hot holes are injected when a bias voltage of morethan 5 volts is created at the source 6 and the drain 7. Morespecifically, lines B1 and B2 show currents passing through the controlgate 5 by hot holes, which are generated depending on the potentialdifference between the junction region in which a bias voltage ispresent and the P-well 1 having the potential of 0 volts. The line B1indicates the flow through the substrate and the line B2 indicates theflow through the control gate 5.

Referring to FIG. 10, as the bias voltage applied to the well isincreased, the potential of the source 6 and the drain 7 is increasedthereby increasing the generation ratio of the hot holes and increasingthe speed of the erasure operation. Lines V1, V2, V3 and V4 indicatechanges in the threshold voltage of the flash memory cell as a functionof the erasure time as the bias voltages applied to the well are 6volts, 6.5 volts, 7 volts and 8 volts, respectively. As can be seen, agreater number of holes are generated as the higher bias voltages areapplied to the P-well 1.

The source 6 and the drain 7 are floated to generate a pure junctioncapacitance. Thus, as an external current flow is not required, theconsumption of current is minimized. Also, as the hot hole injectionmethod is employed and it gives a direction to the vertically formedelectromagnetic field, the capacitance value need not be as high.Therefore, as a reduction in the thickness of the tunnel oxide film 2and the dielectric film 4 is avoided, a degradation in the dataconservation capability is minimized. That is, the disclosed methodminimizes the consumption of current while preserving the thickness ofthe tunnel oxide film 2 and the dielectric film 4. As a result, thespeed of the erasure operation is increased.

An alternate method of erasing a flash memory cell in accordance withthe teachings of the invention can be implemented in a flash memorydevice including PMOS flash memory cells. A PMOS flash memory cell (notshown) generally includes a semiconductor substrate, a stacked gate, asource, a drain, a junction region formed in the semiconductor substratebetween the source and the drain and an N-well. The stacked gateincludes a tunnel oxide film, a floating gate, a dielectric and acontrol gate stacked on the semiconductor substrate. The N-well regionis doped with N-type dopants while the junction region is doped withP-type dopants. The junction region typically has a dopant concentrationthat is greater than 1 E20/cm³ and the N-well region typically has adopant concentration falling within a range of approximately 1 E16/cm³to approximately 1 E18/cm³.

The PMS flash memory cell erasure operation begins with the applicationof a positive bias voltage (+V) to the control gate and a negative biasvoltage (−V) to the N-well while the junction region, that is the sourceand the drain, are floated. The negative bias voltage applied to theN-well typically ranges from approximately five volts to approximately12 volts. N-P junctions are formed between the N-well and the source andbetween the N-well and the drain. The free electrons and the free holesare drawn to interfaces between the source and the N-well and betweenthe drain and the N-well such that a depletion region is formed. As inthe case with the PMOS flash memory cell, a ground voltage is applied tothe N-well while maintaining the positive bias voltage at the controlgate. Some time period after the N-well has discharged, a ground voltageis applied to the control gate.

The above described method of erasing a flash memory cell can be appliedto a memory device having flash memory cells having a triple wellstructure. Referring to FIG. 2, a flash memory cell having a triple wellstructure generally includes both a P-well 1 a and a N-well 1 b formedin a semiconductor substrate 1. The N-well 1 b separates the P-well 1 afrom the P-type semiconductor substrate. The N-well typically has adopant concentration within a range of approximately 1 E18/cm³ to 1E19/cm³ and the P-well typically has a dopant concentration within arange of approximately 1 E16/cm³ to 1 E18/cm³. The triple structureflash memory cell also includes a stacked gate in which a tunnel oxidefilm 2, a floating gate 3, a dielectric film 4 and a control gate 5 arestacked on the semiconductor substrate. A source 6 and a drain 7 areformed in the semiconductor substrate on either side of the stackedgate. The tunnel oxide film 2 typically has a thickness that fallswithin a range of approximately 100 Δ to approximately 200 Δ.

In this approach, the erasure operation begins with the application of anegative bias voltage to the control gate 5 and a positive bias voltage(+V) to the P-well 1 a while the N-well and the junction region arefloated. A forward P-N junction diode is formed between the P-well 1 aand the N-well 1 b, thereby changing the N-well 1 b into a positivepotential. When a voltage of 0 volts is then applied to the P-well 1 a,the voltage at the source 6 and the drain 7 gradually discharges to 0volts. An electromagnetic field is formed between the N-well 1 b and theP-well 1 a as well as between the P-well 1 a and the source 6 andbetween the P-well and the drain 7 such that hot holes are generated.The hot holes generated by the junction capacitance in addition to thehot holes generated by the well capacitance increases the amount of thehot holes injected into the floating gate and therefore furtherincreases the erasure speed. As can be seen in FIG. 11, a leakagecurrent through the control gate 5 can be generated even at a smalljunction potential condition. The line S1 illustrates the drain current,with the N-well and the junction region being floated while the line S2illustrates the drain current, with only the junction region beingfloated. The line S3 illustrates the leakage current through the controlgate, with the N-well and the junction region being floated, while theline S4 illustrates the leakage current through the control gate, withonly the junction region being floated.

A parasitic junction capacitor is generated during the erasure process,so that hot holes are generated without additional flow of current beinggenerated. The generated hot holes are injected in to the floating gate3, thereby increasing the erasure speed of the flash memory cellTherefore, as the erasure speed of the flash memory cell is increased,the thickness of the tunnel oxide film 2 can be increased. Theelectromagnetic field itself formed by the charges trapped in the tunneloxide film 2, reduce the loss of data. Also, immunity against physicaldamage generated by the trapped charges in the tunnel oxide film 2 canbe increased and the breakdown of the tunnel oxide film 2 generated bythe programming and the erasure operation can be prevented.

Also, as the thickness of the dielectric film 4 is increased, thecapacitance between the floating gate 3 and the control gate 5 can bereduced and the loss of data associated with differences in theconcentrations of the charged electrons can be prevented therebyimproving the data conservation capability of a flash memory celldevice.

Although the teachings of the present invention have been illustratedwith reference to particular examples, those having ordinary skill inthe art will appreciate that the scope of this patent is not limited tothose examples. On the contrary, this patent is intended to cover anyand all methods falling within the scope of the appended claims.

1. A method of erasing a flash memory cell including: (a) asemiconductor substrate, (b) a gate, (c) a source, (d) a drain and (e) awell, the gate including: (1) a tunnel oxide film, (2) a floating gate,(3) a dielectric film and (4) a control gate stacked on thesemiconductor substrate, the method comprising the steps of: applying anegative bias voltage to the control gate; applying a positive biasvoltage to the well; creating a positive bias voltage in the source andthe drain; applying a ground voltage to the control gate; andsimultaneously floating the well, the source and the drain at a time. 2.The method of erasing a flash memory cell according to claim 1, furtherincluding the step of, if the flash memory cell is not erased,sequentially applying a ground voltage to the well, to the control gate,and to the source and the drain.